Optical and Surface Morphological Properties of Polarizing Films

Langmuir 2004, 20, 95-100

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Optical and Surface Morphological Properties of Polarizing Films Fabricated from a Chromonic Dye by the Photoalignment Technique Christian Ruslim,*,† Masanori Hashimoto,‡ Daisaku Matsunaga,‡ Takashi Tamaki,† and Kunihiro Ichimura§ Institute for Materials and Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, Functional Chemicals Research Laboratories, Nippon Kayaku Co., Ltd., 3-26-8 Shimo, Kitaku, Tokyo 115-0042, Japan, and Research Institute for Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Received July 27, 2003. In Final Form: October 8, 2003 The inherent chromonic lyotropic liquid crystalline properties of a dye have been manipulated to fabricate multi-axial micropolarizing thin films by means of the photoalignment technique. The dye aqueous solution is deposited on a photopatterned polymer film to result in the macroscopic alignment of the dye molecules, followed by drying at ambient temperature. The solid polarizing dye layers thus produced exhibit very a high contrast ratio and degree of polarization in the region of visible light. Addition of a small amount of surfactant to the dye solution is a prerequisite for the generation of a nematic chromonic phase and for the formation of homogeneous thin dye layers on the polymer film. The correlation between the optical and surface morphological properties of the dye layers is discussed.

Introduction Tremendous efforts have been devoted recently on the manipulation of noncovalent molecular architectures to supramolecular systems that have device functionalities. Hydrogen bondings, ionic or electrostatic interactions, and hydrophobic forces are among the driving forces that build up such supramolecular systems.1 Controlling the nature of molecular self-assembly is one of the challenges in science and technology for the creation of new molecular devices. Some water-soluble dyes spontaneously assemble themselves into a unique superstructure, namely, the chromonic lyotropic liquid crystal (CLLC).2 This type of lyotropic liquid crystal is different from those of surfactant or amphiphilic molecules generally constructed from aliphatic chains. They are planklike in structure and possess conjugated aromatic cores with peripheral hydrophilic or ionic units that make them soluble in water. The strong π-π stacking of the aromatic parts has been described as the main driving force for their molecular self-assembly. Therefore, the assembly of CLLC molecules can adopt rigid columnar, hollow round columnar, hollow square columnar, and brickwork layer structures.3-5 The orientation of such aggregates gives rise to new electrooptical properties. Thus, for example, the application of stimulations such as light and electric or magnetic field to the supramolecular boundary conditions can alternate the output properties. * Author to whom correspondence should be addressed. † National Institute of Advanced Industrial Science and Technology. ‡ Nippon Kayaku Co., Ltd. § Science University of Tokyo. (1) See for example: (a) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001, 101, 4071. (b) Kato, T. Science 2002, 295, 2414. (c) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. (2) Lyndon, J. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 2B, p 981.

It has been reported that CLLC solutions form anisotropic films under mechanical shear flow as the result of the induced ordering of the molecular aggregates.6,7 The dried solid anisotropic dye layers have great potential as cost-effective polarizers. Our systematical studies on the exploitation of photocontrol alignment of liquid crystals have encouraged and successfully led us to the control of anisotropy of both lyotropic dye solutions8 and dye solid layers9 using the photoalignment technique. Compared to the mechanical shear flow, this technique has an advantage of the capability to produce multiple orientations of dyes embedded on a single film. In our recent preliminary communication, it has been demonstrated that a micropatterned polarizing element (MPPE) with a high dichroic ratio, contrast ratio, and degree of polarization (18, 0.85, and 0.91, respectively) in the visible light region has been produced using a dye, C.I. direct blue 67 (B67), on an azobenzene-containing polyamide thin film.10 This optical element is favorable for use in binocular disparity stereoscopic display systems. In this article, we present the detail of the formation of the macroscopic (3) (a) Attwood, T. K.; Lydon, J. E. Mol. Cryst. Liq. Cryst. 1984, 108, 349. (b) Attwood, T. K.; Lydon, J. E. Mol. Cryst. Liq. Cryst. Lett. 1986, 4, 9. (c) Attwood, T. K.; Lydon, J. E.; Jones, F. Liq. Cryst. 1986, 1, 499. (4) (a) Tiddy, G. J. T.; Mateer, D. L.; Ormerod, A. P.; Harrison, W. J.; Edwards, D. J. Langmuir 1995, 11, 390. (b) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. 1996, 100, 2310. (c) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. Faraday Discuss. 1996, 104, 139 (5) Ruslim, C.; Matsunaga, D.; Hashimoto, M.; Tamaki, T.; Ichimura, K. Langmuir 2003, 19, 3686. (6) (a) Iverson, I. K.; Casey, S. M.; Seo, W.; Tam-Chang, S.-W. Langmuir 2002, 18, 3510. (b) Tam-Chang, S.-W.; Seo, W.; Iverson, I. K.; Casey, S. M. Angew. Chem., Int. Ed. 2003, 42, 897. (7) Sergan, T.; Schneider, T.; Kelly, J.; Lavrentovich, O. D. Liq. Cryst. 2000, 27, 567. (8) (a) Ichimura, K.; Momose, M.; Fujiwara, T. Chem. Lett. 2000, 1022. (b) Ichimura, K.; Fujiwara, T. J. Mater. Chem. 2002, 12, 3387. (c) Ichimura, K.; Fujiwara, M.; Momose, M.; Matsunaga, D. J. Mater. Chem. 2002, 12, 3380. (9) (a) Ichimura, K.; Momose, M.; Kudo, K.; Akiyama, H.; Ishizuka, N. Langmuir 1995, 11, 2341. (b) Ichimura, K.; Momose, M.; Kudo, K.; Akiyama, H.; Ishizuka, N. Thin Solid Films 1996, 284-285, 557. (10) Matsunaga, D.; Tamaki, T.; Akiyama, H.; Ichimura, K. Adv. Mater. 2002, 14, 1477.

10.1021/la035366e CCC: $27.50 © 2004 American Chemical Society Published on Web 12/03/2003

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Figure 2. Illustration of the molecular self-assembly of planklike molecules that construct columnar structure of CLLC (the horizontal arrows represent the increase of concentration) and a patterned polarizing dye layer (100-µm stripes, observed under a single polarizer) produced with the procedure described in the text. The bright and the dark stripes of the pattern correspond to the columnar aggregates lying parallel and perpendicular to the polarization direction of the polarizer (double-headed arrow), respectively.

The chemical structures of the dye (B67) and the photoaligning azobenzene polymer (MNC10-PAM) are illustrated in Figure 1. The dye forms unimolecular stacking columns that contribute to the generation of both nematic (N) and middle or hexagonal (M) phases upon addition of a small amount of surfactant (Figure 2). Details

about the structural characteristics in aqueous solutions have been reported earlier.5 The MNC10-PAM polymer belongs to “aminoazobenzene” type azobenzenes, characterized by the close lying of the π-π and the n-π transition moments, and the fast thermal back cis-to-trans relaxation.12 This characteristic could be an important factor to improve the energy efficiency in the course of the photoaligning and photopatterning processes to produce micropatterned polarizers. Linearly polarized light (LPL) was exposed to the MNC10-PAM films on glass substrates to orient the azo moieties in the direction perpendicular to the polarization plane of the LPL (step 1). To obtain multi-axes orientation, the polarization plane of LPL was switched 90°, and then, a photomask was placed between the light source and the polymer film for the successive LPL irradiation (step 2). The deposition of the dye solutions (typically 5 wt %) on the phototreated polymers was performed using a metal roll-coater. After drying at ambient temperature, patterned solid dye layers were produced, an example of which is shown in Figure 2. The dye deposition process can also be performed using a spin coater, as was done in our earlier reports; however, the alignment quality of the resulting solid dye layers was not satisfactory.9 It is to be mentioned that the deposition was done by using the isotropic dye solution. In this manner, the flow-induced alignment is mostly negligible because the dye layers remain still isotropic just after the deposition. Slow evaporation of the water under about 60% relative humidity appeared to be important for the information inscription from the azobenzene polymer to the upper dye layers that eventually transform to the CLLC phase until solidification. We have also confirmed that photoinduced alignment overwhelms flow-induced alignment by changing the rolling direction during deposition. Nevertheless, this is valid only when the dye solution is isotropic. When we used the CLLC phase solution for deposition, the photoinduced alignment of the azobenzene film was not transferred properly to the dye layer. Instead, the flow-induced alignment became dominant in this case. The typical optimized conditions for the fabrication of photoaligned dye solid films using the roll-coater give an excellent optical performance, as shown in Figure 3. The polarized UV/vis spectra suggest that there is almost no absorption of light propagating parallel with the polarization plane of the actinic LPL in the photoalignment process. Quantitatively, the alignment of the dye molecules in the solid layers is represented by the order parameter (S) and the degree of polarization (P), defined as follows.

(11) Matsunaga, D.; Tamaki, T.; Ichimura, K. J. Mater. Chem. 2003, 13, 1558.

(12) Rau, H. In Photochromism: Molecules and Systems; Duerr, H., Bouas-Laurent, H., Eds.; Elsivier: Amsterdam, 1990; p 165.

Figure 1. Structures of the photoaligning polymer, MNC10PAM, and the chromonic dye, B67.

anisotropy of the dye layers, especially on the effect of the addition of a small amount of surfactant, and discuss the correlation between the optical and morphological properties of the oriented films. The thermal stability of the aligned dye layers, which is of great importance in applications, is also investigated. Experimental Section Materials. The chromonic dye, C.I. direct blue 67 (Nippon Kayaku Co., Ltd.) was recrystallized from ethanol/water. Neutral ultrapure water of Millipore grade (18 MΩ cm-1) was used to prepare the dye solutions. Anionic surfactant solution, Emal20C, was purchased from Kao Co. and used as received. This surfactant solution contains 25.5 wt % poly(oxyethylene) lauryl ether sodium sulfonate. The aqueous dye and surfactant mixture was stirred at room temperature until no insoluble particle was observable under a microscope. The azobenzene polymer MNC10-PAM was synthesized according to the procedure reported before.11 Instrumentation. Photoaligning azobenzene polymer films were prepared by spin-coating a 2 wt % NMP solution of MNC10PAM on clean glass substrates (7.6 × 5.2 cm2), followed by drying at 100 °C for 2 h. The spin-coated azobenzene films were exposed to linearly polarized visible light from a 500-W high-pressure mercury lamp (Ushio) through glass filters. Detail of the irradiation condition has been reported.10 Optical textures of the LLC solutions were observed using a polarized optical microscope BX50 (Olympus), connected to a digital camera PDMC Ie/OL (Olympus) and a hot-stage LK-600PM (Linkam). Polarized UV/ vis absorption spectra were recorded on a polarization microspectrometer MCPD-3000 (Otsuka Electronics Co., Ltd.). A hot stage was mounted on the sample holder for temperaturecontrolled measurements. The contact angle was measured using a contact angle meter Face CE-X (Kyowa Interface Science Co., Ltd.). Atomic force microscopy (AFM) topological images were recorded on a PicoSPM model MS300 (Molecular Imaging, Ltd.) at room temperature in the contact mode with a scanning speed of 1.0 Hz.

Results and Discussion

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Figure 4. Effect of the surfactant/dye weight ratio of the photoaligned dye layers on the S and P parameters calculated at λmax of the dye. Here, the concentration of the surfactant is calculated as the weight concentration of poly(oxyethylene) lauryl ether sodium sulfonate in Emal20C. The vertical lines describe the standard deviations of the measured samples (typically 8 data from 2 separate films). Figure 3. (a) Definition of the MNC10-PAM film geometry during irradiation with LPL, (b) polarized UV/vis spectra of a photoaligned dye layer deposited from a 5 wt % B67 aqueous solution containing 0.2 wt % Emal20C, and (c) order parameter (S) and degree of polarization (P) of part b.

D-1 D+2

(1)

T90 - T0 T90 + T0

(2)

S) P)

Here, D ) A90/A0 is the dichroic ratio and T is the transmittance of monitoring LPL with the subscript, indicating the directions defined in Figure 3a. Around the λmax of the dye, S and P as high as 0.89 and 0.95, respectively, are obtained (Figure 3c). These values are significantly improved compared to the photoalignment using poly(4-methacryloyloxyazobenzene) films, which produced S in the range of 0.1-0.57.9,13 The alignment features here are also comparative to those obtained by the shear-flow technique. One of the most important factors that decisively determine the optical performance of the dye layers is the addition of a small amount of surfactant to the dye solutions. It has been shown experimentally that an anionic surfactant, Emal20C, promoted the formation and stabilization of the N phase of the CLLC dye solutions,5 though there is still no clear explanations at the molecular level about the role of the surfactant molecules in the vicinity of the dye aggregates. There is an optimal addition amount of the surfactant to give high-quality dye layers. Figure 4 shows the average values of S and P of the photoaligned dye layers as a function of the surfactant/ dye weight ratio. In this figure, the surfactant concentration is calculated as the concentration of poly(oxyethylene) lauryl ether sodium sulfonate, the active agent in (13) Fujiwara, M.; Master Thesis, Tokyo Institute of Technology, Tokyo, Japan, 1997.

Figure 5. Static contact angle of 5 wt % B67 aqueous solutions containing different concentrations of Emal20C on spin-coated MNC10-PAM films. The corresponding surfactant/dye ratio is also shown in the upper abscissa.

Emal20C, because the solvent (water) in Emal20C was evaporated during the process. All the experiments here adopted the dye concentration of 5 wt %, which has been found to be the optimal concentration for the formation of polarizing dye films. A lower dye concentration gave dye alignment with a lower contrast ratio. On the other hand, a high concentration of dye solution led to CLLC phases. Deposition of such dye solutions produced flowinduced alignment and, thus, destructed the photoinduced alignment on anisotropic azobenzene thin films. It can be clearly observed in Figure 4 that the surfactant/dye weight ratio as small as about 0.01 gave maximal S and P. It can also be confirmed here that the standard deviation tends to broaden as the S and P shift away from their maximal. Without any surfactant, it was almost impossible to get a homogeneous dye layer because the polymer surface repelled the dye solution, resulting in the random distribution of dye domains over the polymer layer. This is mainly due to the hydrophobic nature of the polymer films. Upon addition of a small amount of surfactant, the quality of the overall layer formation increases dramatically. See Supporting Information for the micrographs of the pho-

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Figure 6. AFM topological images (10 × 10 µm2) of the photoaligned dye layers with surfactant/dye ratios of (a) 0, (b) 4.1 × 10-3, (c) 1.0 × 10-2, (d) 2.0 × 10-2, (e) 4.1 × 10-2, and (f) 1.0 × 10-1. The cross-sectional profiles are shown below the corresponding images.

toaligned solid dye layers having different surfactant/dye ratios. The wettability of the polymer films when contacted with the dye solutions with a different content of surfactant is eminently observable through the static contact angle measurement, as shown in Figure 5. Without surfactant, the contact angle of an aqueous 5 wt % dye solution was 72°, similar to that of pure water. The contact angle reached a minimum value at a surfactant concentration of about 0.2 wt % and remained constant with the increase of surfactant. This is in accordance with the fact that the deposition of the dye solution containing a surfactant concentration of more than 0.2 wt % solved the problem of repellence and gave homogeneous dye layers in appearance. However, there is an optimal surfactant concentration needed to produce photoaligned dye layers with high optical performances, as described in Figure 4. Exceeding this value will destruct and eventually destroy the alignment of the dyes. Indeed, we have also demonstrated that the high surfactant loading brought about phase separation of the CLLC phase.5 This micro phase separation becomes more evident through surface morphology analysis of the solid dye layers using AFM. Using contact-mode AFM, we investigated the surface morphology of the dye layers as a function of the surfactant content. The results are illustrated in Figure 6. Obviously, the aggregate size and shape are influenced significantly by the surfactant content. Without any surfactant content (image a), the dye layer surface is quite flat and no pronounced aggregate shape could be concluded. At surfactant/dye ) 0.0041 (image b), the surface appeared to be rugged and rather elongated aggregates are observed.

The cross-sectional profile suggests a surface roughness of 10-20 µm. Image c corresponds to the dye layer with the highest S and P values. Notice that detailed fiberlike aggregates gather into bundles that are stretched in the direction perpendicularly to the transition moment of the dye molecule (θ ) 90°). Therefore, the fiberlike structure can be assumed as the longitudinal assembly of the columnar structure of the CLLC dyes. Quite similar aggregate morphologies were also reported for another chromonic dye in its anisotropic phase.14 Important to notice are the images with higher content of surfactant. For example, image f (surfactant/dye ) 0.1) is characterized by the appearance of some micrometer-sized spindleshaped holes orienting parallel with the direction of the fiberlike aggregates. The micro phase separation in the CLLC phase during the solvent evaporation process is thought to be responsible for the emergence of these holes. Quantitative image analysis was performed to draw comprehensive information about the morphological structures in Figure 6. The root-mean-square roughness (R) of a given surface is calculated through

R)

x

N

(zj - zav)2 ∑ j)1 N

(3)

where zav and zj are the average feature height (z) within (14) Imae, T.; Gagel, L.; Tunich, C.; Platz, G.; Iwamoto, T.; Funayama, K. Langmuir 1998, 14, 2197.

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Figure 8. Effect of heat treatment of the photoaligned dye layers on the S (circle) and P (square) parameters. Each temperature was maintained for 5 min before any measurement.

Figure 7. (a) Logarithmic plots of the cell area (ca) against the surface area (sa) according to formula 4 for photoaligned dye surfaces (10 × 10 µm2) with surfactant/dye ratios of 0 (circle), 4.1 × 10-3 (square), 1.0 × 10-2 (diamond), 2.0 × 10-2 (triangle), 4.1 × 10-2 (inverted triangle), and 1.0 × 10-1 (cross). (b) Effect of the surfactant/dye ratio of the photoaligned dye layers on the roughness (R; dotted lines) and fractal dimension (f; solid lines) for image area sizes of 10 × 10 µm2 (circle), 20 × 20 µm2 (square), and 30 × 30 µm2 (triangle). The cross marks in the figure are those for a dye layer produced by the spin-coating method on a nonirradiated MNC10-PAM film.

the surface area and the current z value, respectively, and N is the number of data.15 Because R represents only the standard deviation of the z value, another parameter to discriminate the surface morphology in the x and y (in-plane) directions from R is required. Fractal dimension (f) analysis is adopted to extract the geometric complexity of the surface.15

sa ) mca - f

(4)

It is based on successive steps of dividing a surface into triangular cells (ca). In each step, each triangle area is adjusted to cover the surface in the maximum coverage area (sa). The resulting surface area is then calculated. The fractal dimension is obtained from the steep slope of the logarithmic plots of ca against sa. For a threedimensional surface, f varies from 2.00 for a smooth surface to 3.00 for an infinitely complex surface. Figure 7a shows the plots according to formula 4 for surfaces of 10 × 10 µm2 size. The f values were then plotted in Figure 7b together with the R values for the corresponding surfaces in three different sizes, that is, 10 × 10 µm2, 20 × 20 µm2, and 30 × 30 µm2, respectively. From this figure, it is found that the dye layers have surface roughnesses of 10-30 nm, which are relatively smooth. The fractal dimension f spanned in the range of 2.302.40. The layer having the best alignment properties (surfactant/dye ) 0.01) seems to be characterized by the combination of a low R and high f. This is more conclusive (15) NanoScope Scanning Probe Microscope: Command Reference Manual Version 4.22ce; Digital Instruments, 1997.

for homogeneous surfaces, that is, surfaces without any phase separation, as described in Figure 6. It should be kept in mind that, for surfaces with a large surfactant/ dye ratio, the f parameter may appear less reliable and may exhibit quite a strong dependency on the size of the analyzed area (Figure 7b) as a result of the inhomogeneous distribution of the spindle-shaped holes on the surface. The polymer surfaces themselves (without deposition of dye layers) possessed R and f of 10.6 and 2.20, respectively, indicating that the polymer surfaces are relatively flat and smooth. These morphological parameters were essentially not altered after the polymer films were subjected to LPL irradiation to orient the azobenzenes (10.4 and 2.25 respectively, after LPL irradiation). The effect of mechanical contact between the roll-coater and the dye surface could be problematic because they were in contact during the deposition process. Therefore, we produced a test sample of a dye layer deposited on a polymer film using the spin-coating technique, where no physical contact to the top surface of the dye layer occurred, and investigated the surface morphology. The cross marks in Figure 7b are the morphological parameters for this sample (R ) 7.5 and f ) 2.41). They are obviously comparable to those produced by a roll-coater so that it is reasonable to say that the effect of physical contact mentioned previously was very small and negligible. This may also reflect the fact that the deposition was carried out in the isotropic fluid state of the dye solutions. The photoaligned dye layers are also required to have thermal durability for practical application. A representative dye layer was then subjected to continuous heating to 180 °C. During this process, the S and P parameters were maintained stably (Figure 8). Only a very slight decrease of the parameters was observed. Additionally, the morphological structure was unaltered after the heat treatment. This means that the aligned dye layers possess excellent thermal durability, a prerequisite for device application. Conclusions Macroscopically aligned dye layers produced from B67 and MNC10-PAM polymer as a photoaligning layer were fabricated using the photoalignment technique. It is demonstrated that the addition of a small amount of surfactant affects significantly the optical and structural morphology of the resulting dye layers. The surfactant seemed to have dual functions, that is, generating and stabilizing the nematic phase of CLLC and improving the compatibility of the dye solution on the anisotropic polymer surfaces during the deposition process. Optimization of the material and fabrication process has significantly

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improved the optical properties of the dye layers (S ) 0.89, P ) 0.95) to a level comparable to the stress-flow process in CLLC phase. Furthermore, the surfaces of the dye layers are microscopically homogeneous. At the nanoscale level, the optimized aligned dye layer is characterized by a low surface roughness and high geometric complexity. The dye layers can be patterned easily and in a macroscopically large size. Essentially, they possess high thermal stability and are, therefore, suitable for application as MPPEs. More efforts devoted to the optimization of materials and oriented toward this application are in progress.

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Acknowledgment. C.R thanks NEDO (New Energy and Industrial Technology Development Organization) for the NEDO fellowship. The authors also thank Dr. Haruhisa Akiyama from AIST for the instruction and discussion of AFM. Supporting Information Available: Microphotographs of the photoaligned solid dye layers with surfactant/dye weight ratios of 0, 0.01, and 0.1, respectively, observed under a single polarizer with the polarization direction perpendicular to that of the actinic LPL (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA035366E